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标题图片：http://en./wiki/File:Weltkarte_der_Netzspannungen_und_Netzfrequenzen.svg 目前世界各国市电（AC交流电）的电压各不相同。一般都是是110V或者220V。交流电的频率一般是50Hz。只有一半国家使用60Hz。美国使用110V60Hz的交流电。 各国电压和频率列表： List of Worldwide AC Voltages and Frequencies.
这节课会解答这些问题。 当时为什么选择这些电压和频率家用和商用供电最初使用的是直流电（DC）然后才变成了交流电（AC）。最开始使用的电压是110V，然后变成了240V，又变成110V，最后又变成了220V。大部分地区的频率由最开始的60Hz后来变成了50Hz。 特斯拉与交流电在电力使用的早期历史中，爱迪生（Thomas Edison）的通用电力公司在美国使用110V供给直流电。然后特斯拉（Nikola Tesla）发明了3相的240V交流电。三相的意思是有三个不同相位的电力合并在一起以减弱供电中的电压波动。他计算得到1秒钟60个周期是最高效的。特斯拉后来以安全的名义把电压降低到了110V。 欧洲使用50Hz在Westinghouse 公司的资助下，特斯拉的交流电体系成为了美国的供电标准。但是与此同时，德国AEG公司开始提供电力并且在欧洲处于垄断地位。他们决定用50Hz代替60Hz，因为他们用十进制（美国使用12进制的英制），这样计算更方便，不过他们还是保留了110V的电压。 不幸的是，50Hz的交流电效率很差远不如60Hz。由于50Hz的发电机的效率比60Hz低了20%。50Hz的电力传输效率低了10到15%。50Hz的变压器需要更大的绕组，50Hz的电动机效率也比60Hz的低。因为他们采用了低频率导致他们必须花更多钱来解决电力损失和更大的发热量的问题。 欧洲使用220V直到二战后的20世纪50年代欧洲还是使用110V，然后他们就改为220V来 电力传输效率。英国不仅改为220V,他们为了和欧洲大陆保持一致，把频率改为了50Hz。因为二战后欧洲还没有太多的电力设施，所以这次改变成本并不是很大。 美国依然使用60Hz和110V美国也考虑改用220V来提供市电，但是有感觉这样太浪费了，因为有很多电气已经在用110V的电压了。最后妥协的方案是：220V电力进入家庭后会分为110V来给大部分电器供电，但是像电炉、电烘干机就用220V。 特例有些国家没有统一标准。 巴西在巴西，大部分地区使用110V到127V之间的交流电。但是大部分旅馆用220V。在巴西利亚（Brasilia）主要使用220-240V电压。 日本在日本，所有地区的电压都一样，但是不同地区频率不一样。日本东部，包括东京，使用50Hz。西部包括大阪和京都使用60Hz。 原因是二战后，日本东部和西部的电力系统的重建任务分别由英国和美国接手了。当时英国刚刚跟随欧陆使用了240V50Hz的交流电，他们很奇怪的使用了100V50Hz，因为美国用的是60Hz。 你去其他国家会遇到什么？============= 这段意义不大，暂时不翻译。 Bringing an electrical appliance from one country to another may require some special converters, transformers and adapters to allow the appliance or device to work properly. ConvertersConverters are typically used to decrease the AC voltage from 220V to the 110V level needed by the appliance. They are only used for simple electrical products such as hair dryers, steam irons, shavers, or small fans. They are only used for short periods of time, can only be used for ungrounded appliances, and must be unplugged from the wall when not in use. Converters cannot be used by electronic devices such as radios or computers. A transformer is used for those devices. The reason is that a converter simply cuts the AC sine wave in half, reducing the voltage. Electronic devices need the full sine wave to function properly. Some converters will also change AC to DC. An example is converting 120V AC to 12V DC. TransformersTransformers are used to increase or decrease the voltage and should be used with electronic devices such as radios, televisions, computers and other devices having electronics circuitry. Transformers are more expensive than converters. They can also be used with electric appliances and may be operated continually for many days. A device like a hair dryer does not have any electronic circuitry. It simply has a heatingr element and electric fan, so it can use either a converter or transformer. Dual voltage devicesSome devices have a built-in converter or transformer, such that they are called dual voltage devices. Most laptop battery chargers and AC adapters are dual voltage, so they can be used with only a plug adapter for the country you are visiting. Plug AdaptersOutlet plugs are different in the various countries. Plug adapter must often be used when visiting a different country. These adapters do not convert electricity. Rather, they simply allow a dual voltage appliance, transformer or converter from one country to be plugged into the wall outlet of another country. Frequency differenceConverters and transformers only change the voltage and not the frequency. The result is that a motor in a 50Hz appliance will operate slightly faster on 60Hz electricity. Likewise, a clock made for 60Hz will run slower in a country using the 50Hz frequency. Most modern electronic equipment like computers, printers, DVD players and stereos are usually not affected by the frequency difference. ============== 总结世界各国的交流电的电压和频率各不相同。大部分是220V50Hz。20%的国家使用110V60Hz供给市电。220V60Hz是最高效的，但是只有极少数国家采用这种配置。美国用110V60Hz。 下边也不翻译 ============== Electrify society by applying your knowledge of science Resources and referencesFollowing are some resources on this subject. Web sitesWorld Standards - Electricity standards Elements of AC Electricity - Basic electronics tutorial site DC and AC Electricity Resources BooksBasic Electricity by Bureau of Naval Personnel; Dover Pubns; (1970) $14.95 - Provides thorough coverage of the basic theory of electricity and its applications |
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